In light of such a large range of fuel qualities, it is no longer possible with regard to the operation of an internal combustion engine at every engine operating point to find suitable parameters for the open-loop or closed-loop control of the combustion in the cylinders, as for example the quantity of injected fuel, the point in time of pilot and main injections, the rail pressure of a common rail fuel injection system, the quantity of fresh air supplied to the cylinders, the supercharging pressure in supercharged internal combustion engines, the throttle valve position etc., with which good results with regard to emissions and drivability of the motor vehicle can be obtained for all of the fuel qualities.
That is why practical methods are needed, with which the ignitability of the fuels used in the operation of the internal combustion engine, respectively the motor vehicle, can be ascertained with sufficient accuracy.
Such methods have already been proposed, wherein the quality and particularly the ignitability of the fuel being used is derived from the progression of the cylinder pressure. What is considered a disadvantage in these methods is, however, that the installation of an additional cylinder pressure sensor is required and that the measurement of the cylinder pressure can only be performed in a special operating mode of the internal combustion engine.
Based on this fact, the task underlying the invention is to improve a method of the kind mentioned at the beginning of the application to such an extent that the determination of the ignitability of fuel with an unknown fuel quality can be performed during the normal operation of the internal combustion engine and without necessitating the use of additional sensors.